S. Kyyrö: Pressurized hot water extraction as a modification method for solid wood

Abstract: 

Pressurized hot water extraction (HWE) of the wood utilizes high-temperature water kept in liquid form with pressure. The simultaneous obtainment of hemicellulose-based carbohydrates and treatment of the solid wood is a notable advantage of HWE treatment, but it has only limited effects on the material properties compared to the thermal modification traditionally used to produce durable timber. Finding solutions to the limitations could enable the development of novel wood products. In this thesis, a broad range of HWE treatment conditions was applied to wood samples of varying sizes. Both the effects of increasing wood size and important material properties such as hygroscopicity and resistance against wood-inhabiting fungi were studied. The aim was to improve understanding of the properties of HWE treated wood in view of its potential in engineered products and to evaluate the underlying mechanisms.

The studies showed that the HWE treatment efficiency differences between sample sizes were mainly caused by the higher amount of water and acetone soluble degradation products that remained in wood with larger dimensions. Furthermore, water soluble degradation products accumulated on the wood surface and were removable via leaching or surface planing. The removal of these degradation products with either approach was shown to improve the resistance of HWE treated wood against discolouration by mould fungi.

Prior to first drying, HWE treatment did not considerably change the water-saturated dimensions of the wood as degraded components were replaced with water. The first drying and resoaking resulted in a shrinkage in the saturated dimensions. This restriction in the reswelling of the cell walls was enhanced by a longer treatment time. Hydroxyl accessibility of the wood was assessed by measuring the mass loss caused by the reprotonation of deuterated samples. The effects of first drying could not be assigned solely to the variations in the sorption site density. The results suggested that the cell wall space formed during HWE treatment partially closes due to first drying and does not fully reopen upon resoaking. Therefore, diffusion of modification agents and subsequent esterification was tested as an approach to improve the properties of HWE treated wood. Successful esterification and cell wall bulking were confirmed, but the method's efficiency in improving durability varied between tested wood-decaying fungi.

The results of this thesis offer new insights into the characteristics of HWE treated wood and potential solutions to improve its dimensional stability and resistance against wood-inhabiting fungi. The thesis also provides the groundwork for future studies regarding the material properties of wood modified with HWE treatment.

Follow the remote defence: Remote participation

Opponent: Professor Henrik Heräjärvi, University of Eastern Finland, Finland

Supervisor: Professor Lauri Rautkari,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Pressurized hot water extraction as a modification method for solid wood